Radio frequency identification (RFID) technology has gained tremendous popularity as a device for storing and transmitting information. RFID technology utilizes a tag transponder, which is placed on an object, and a reader, also referred to herein as an interrogator, to read and identify the tag. RFID technologies are broadly categorized as using either “active” tags or “passive” tags. Active tags have a local power source (such as a battery) so that the active tag sends a signal to be read by the interrogator. Active tags have a longer signal range. “Passive” tags, in contrast, have no internal power source. Instead, passive tags derive power from the reader, and the passive tag re-transmits or transponds information upon receiving the signal from the reader. Passive tags have a much shorter signal range (typically less than 20 feet).
Both categories of tags have an electronic circuit that is typically in the form of an integrated circuit or silicon chip. The circuit stores and communicates identification data to the reader. In addition to the chip, the tag includes some form of antenna that is electrically connected to the chip. Active tags incorporate an antenna that communicates with the reader from the tag's own power source. For passive tags, the antenna acts as a transducer to convert radio frequency (RF) energy originating from the reader to electrical power. The chip then becomes energized and performs the communication function with the reader.
On the other hand, a chipless RFID tag has neither an integrated circuit nor discrete electronic components, such as the transistor or coil. This feature allows chipless RFID tags to be printed directly onto a substrate at lower costs than traditional RFID tags.
As a practical matter, RFID technology uses radio frequencies that have much better penetration characteristics to material than do optical signals, and will work under more hostile environmental conditions than bar code labels. Therefore, the RFID tags may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. RFID tags may be used in managing inventory, automatic identification of cars on toil roads, security systems, electronic access cards, keyless entry and the like.
Antennae are an element of RFID tags that are typically prepared via stamping/etching techniques, wherein a foil master is carved away to create the final structure. For example, each RFID chip is encoded by etching a specific set of resonant structures into a conductive film. These structures create a frequency dependent antenna load which can be used to encode data that is read by observing the reflected or re-irradiated pulse responsive to a broad band interrogating pulses. This process increases the cost of such tags by requiring that each chip must be made individually, currently by laser etching, which is an expensive process.
An alternative approach for printing customized RFID antennas having a unique spectral signature from the “bottom up” have been attempted. Such method involves by printing directly on a substrate using a conductive metal ink. Inkjet printing has been identified as one possible technology for this purpose because it can easily be either used alone to print structures and/or integrated with other processes and machines. For example, hybrid printing technology combines analog printing processes (such as offset lithography, flexography, etching, and letterpress printing) which may be used for printing wire segments of an RFID antenna, with digital printing processes (such as inkjet printing) for printing a conductive ink to interconnect the wire segments. However, inkjet printing has proved unreliable for printing customized RFID antennas because the resolutions of inkjet printheads are inadequate and because the patterns of conductive materials formed by inkjet printheads result in non-uniform printed structures of varying interconnectivities which are unacceptable for RFID structures. Additionally, to print customized RFIDs inexpensive, the conductive particles of the inks must also be inexpensive. However, such particles are usually low cost, low-conductivity materials of varying particle size which affect the overall interconnectivity of the printed structures. On the other hand, conductive ink particles having small particle diameters, having uniform particle diameters, and/or those made of high-conductivity material like gold and silver are expensive, rendering them unusable for providing cheap, customized RFID tags.
Accordingly, inkjet printheads are not suitable for patterning the ink at the scale necessary for encoding individual resonant structures from the “bottom up”. As a result, inkjet technology for printing RFID antennas cannot compete with fully lithographic stamping/etching techniques described above, but lithographic stamping/etching processes are not efficient for use in making customized tags and can be expensive because doing so requires laser ablation steps. Accordingly, a lower-cost and more efficient alternative to conventional for making customized RFID tags would be a welcome addition to the art.